The present disclosure relates to an ink for an ink jet recording apparatus, and also to an image forming method that uses this ink.
In recent years, rapid advances in recording technology have enabled high definition images that compare with silver salt photographs to be obtained, meaning that ink jet recording apparatuses, which form images by ink jet recording methods, are widely used as image forming apparatuses.
There is a strong demand for such an ink jet recording apparatus in which image quality can be maintained while increasing the image forming speed. However, when forming images at high speeds using ink jet recording apparatuses, the recording medium, such as a paper, passes through a pair of discharge rollers and is discharged before the ink can penetrate into the recording medium, meaning that the ink can become deposited (offset) on the discharge rollers. In such cases, image defects readily occur in formed images due to ink that has been deposited on the discharge rollers becoming deposited on the recording medium.
In order to suppress the occurrence of offsetting, reducing the quantity of ink discharged per pixel has been considered. However, forming an image having the desired image density is difficult in such cases. Therefore, in order to suppress the occurrence of offsetting while forming an image having the desired image density, a great deal of consideration has been given to compounds which can be incorporated in an ink in order to improve the penetration of the ink into a recording medium.
With such circumstances in mind, a proposal was made for an ink that contains a compound in which a specific quantity of ethylene oxide and/or propylene oxide is added to a straight chain alkanol as an ink which exhibits excellent penetration into a recording medium (drying properties) and which can form a high quality image (prior art 1).
In addition, inks that contain specified quantities of non-ionic polymers in pigments have also been reported (prior art 2 and 3). Furthermore, an ink to which acetylene glycol (a surfactant) is added has also been proposed (prior art 4).
Here, in order to form a high quality image, it is desirable for dots, which are formed by ink droplets impacting upon a recording medium, to have a sufficiently large dot diameter and to have a shape that is nearly circular. However, the ink in prior art 1 cannot easily form dots that are nearly circular, whereby it is difficult to form a high quality image.
Meanwhile, the inks in prior art 2 and 3 use poly(vinyl alcohol) or polyvinylpyrrolidone, not an acrylic polymer, as a water-soluble polymer (resin), and because these substances cause the surface tension of the ink to increase, inks containing these substances have the problem of poor continuous discharge properties.
Furthermore, in addition to high quality, there are demands for even faster printing speeds in the field of printing these days. In order to achieve higher printing speeds, shortening the drive timing of a recording liquid discharged from a recording head, lining up small recording heads in a band-like pattern and using long line heads have been considered. However, when printing with a line head, problems occur in terms of continuous printing properties, print quality and satellite formation.
First, with regard to continuous printing properties, because continuous printing conditions are such that ink is continuously discharged from heads, problems occur due to jetting twisting or non-discharge caused by ink misting or ink deposits at the edge of nozzle openings and the like.
With regard to print quality (print density), problems relating to print density, strike-through and unevenness in darkness are given as examples of problems that occur in terms of print quality on ordinary paper. Strike-through density increases if the surface tension of the ink is low or if the ink excessively wets the paper. In addition, in cases in which the ink has a high surface tension or in cases in which the wettability to the paper is insufficient, unevenness in darkness can occur, leading to concerns that the print density will be reduced.
Satellites are a problem caused by discharged liquid droplets splitting and the landing position of a single dot becoming misaligned. This is a problem that occurs particularly when the dynamic/static surface tension of the ink is low.
For example, in cases in which the compound disclosed in prior art 1, in which a specific quantity of ethylene oxide and/or propylene oxide is added to a straight chain alkanol, is used in order to address such problems, a balance between print density and continuous printing properties cannot be achieved. In addition, by using a surfactant (acetylene glycol), as is the case with the ink disclosed in prior art 4, the decrease in dynamic surface tension is large, and there are concerns that satellites will occur and that the continuous printing properties will deteriorate. Therefore, it is currently the case that conventional inks cannot satisfactorily address the problems mentioned above.